In obtaining oil from areas where tertiary recovery is necessary, one of the primary requirements is to obtain accurate oil saturation data. Although other methods are utilized, coring is one of the primary methods for determining oil saturation data. In obtaining this data, it is important that it be as accurate as possible since very small inaccuracies can be the difference between profit and a loss in subsequent drilling operations. One reason for the inaccuracies incurred in coring is the result of washing of the cores by drilling mud filtrate which can remove a large fraction of the mobile oil that is present in the cores. This mobile oil is the oil that passes through the core and that provides one indication of future productivity of the well.
To analyze the amount of oil that is contained in a particular soil at a particular depth in the proximity of a subterranean well requires extraction of a sample of the well material. Analysis of this material yields the percent of fluid and/or gas contained therein which is utilized to determine the type of fluid, such as oil, contained therein and the pressure thereof. However, it is important in order to obtain an accurate analysis to extract the core in as intact a condition as possible. Since the fluid and gas are contained in the core material at a pressure dependent upon the depth of the well, extraction of this core to an environment with a lower pressure results in the fluid expanding somewhat and the gas coming out of solution. In addition, the mobile oil contained in the core may also drain or "bleed" out of the core and be lost.
One method for retaining mobile oil is sponge coring which is disclosed in U.S. Pat. No. 4,312,414, issued to the present Applicant. Sponge coring comprises disposing a high porosity sponge on the interior surface of the inner barrel of the well coring apparatus. The core is then forced into the inner barrel with the sponge disposed about the sides thereof. The oil and/or gas contained in the core then "bleeds" into the sponge thereby retaining an accurate profile of the oil along the longitudinal axis of the core.
To obtain proper absorption of the oil that bleeds from the core, it is necessary to have the core disposed adjacent the surface of the sponge in relatively close proximity. To do this, the inner diameter of the sponge coring member is dimensioned to be slightly less than the diameter of the core itself. This allows for a very tight fit therebetween. However, this presents problems such as breakage of the sponge and/or the core. If the sponge breaks, this can result in jamming, that is, the sponge preceding the core up through the inner barrel.
Normally, the sponge is an open celled material that has dead air space which receives the fluid from the core. This presents a problem in that the air volume in the sponge must have a place to escape in order to relieve back pressure that may build up and provide room for the oil. If this pressure remains within the sponge itself, the accuracy of the saturation data can be somewhat inhibited.
In view of the above disadvantages, there exists a need for a sponge coring apparatus that relieves pressure within the sponge during the coring process and minimizes breakage of the sponge as the core proceeds into the inner barrel.